As a molecular switch the ras protein undergoes structural changes that couple recognition sites on the surface of the protein to the guanine nucleotide-divalent metal ion binding site. X-ray crystallographic studies of p21 suggest coordination between threonine-35 and the divalent metal ion plays an important role in these conformational changes. Recent ESEEM studies of p21 in solution, however, place threonine-35 further away from the metal and were interpreted as weak or indirect coordination of this residue. We report high frequency (139.5 GHz) EPR spectroscopy of p21Mn(II) complexes of two alternate guanine nucleotides that probe the link between threonine-35 and the divalent metal ion. In particular, we determine the number of water molecules in the first coordination sphere of the manganous ion to be four in p21Mn(II)GDP and two in p21Mn(II)GMPPNP. The results for GMPPNP (a GTP-analog) are consistent with the number of water molecules predicted by the X-ray structure. While these results rule out indirect coordination of threonine-35, they are consistent with direct, weak coordination of this residue as suggested by Halkides et al. (1994) Biochemistry 33, 4019. The 17O hyperfine coupling constant of H217O is determined to be 2.5 G in the GDP form and 2.8 G in the GTP form. These values are similar to reported values for 17O-enriched aquo- and phosphato-ligands in other complexes of Mn(II). For all of these measurements, the elevated field yields a reduction of Mn(II) EPR linewidths, which enhances the spectral sensitivity to 17O hyperfine broadening.